Numerous inorganic materials have been identified as potential candidates for high-performance photocatalysts. However, their solar-to-energy conversion efficiencies still fail to meet commercial requirements. The main hurdle is the rapid recombination of photoexcited electrons and holes in single-phase materials. A viable predicted approach to suppress charge recombination is coupling two materials to form a two-dimensional (2D) heterostructure that physically separates photoinduced electrons and holes in different layers. In this work, the heterostructure-based paradigm was tested and a scalable solution synthesis of epitaxial ZnO-MoS2 heterostructure was developed. A 2D ZnO-MoS2 heterostructure was synthesized under hydrothermal conditions by stabilizing intermediate Zn-hydroxide states on a functionalized MoS2 surface. Detailed characterization showed the formation of multilayer heterostructure with MoS2 flakes intercalated between large size ZnO plates. The performance of this heterostructure was evaluated using photocatalytic degradation of rhodamine B. A degradation efficiency of 70% was measured within 90 minutes of visible-light irradiation, almost doubling the efficiency of the corresponding single-phase materials or their physical mixtures.
Research Article
Solution synthesis and enhanced photocatalytic activity of layered ZnO and MoS 2 hybrid nanostructure
https://doi.org/10.21203/rs.3.rs-3123105/v1
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2D heterostructure
intermediate state
hydrothermal synthesis
photocatalysis
Semiconductor metal oxides photocatalysts, such as TiO2,(1; 2) ZnO,(3) and SnO2,(4) have been extensively studied because of their non-toxicity, chemical inertness, photostability, and ability to utilize visible and UV light. The main disadvantages that hinder the practical application of single phase semiconductor photocatalysts are the fast recombination of photogenerated electron–hole (e-/h+) pairs and the narrow light-response region due to their relatively large band gaps (e.g., 3.37 eV for ZnO).(5; 6) Transition metal dichalcogenides (TMDs), with direct band gaps ranging from 1 to 2.5 eV, have emerged as promising materials for photocatalysis in the near-infrared to visible range. For example, molybdenum disulfide (MoS2), one of the most stable layered TMDs, is considered an emerging material for efficient photocatalysis.(7) MoS2 nanosheets have a direct band gap of 1.9 eV, appropriate for activating photocatalytic processes by visible light. Further advantages include high in-plane electron mobility, nontoxicity, and earth-abundance. Nevertheless, TMDs also suffer from rapid photoelectron-hole recombination.(8) A strategy for increasing photoelectron and hole lifetimes is to use quantum confinement effects by reducing the dimensionality of metal oxide structures and creating core-shell or layered heterostructures. For example, core-shell TiO2-ZnO spherical particles demonstrated reduced electron-hole recombination and a red shift of the absorption band to the visible region, leading to an enhanced photoconversion efficiency of 0.55%.(9) Computational predictions show even higher potential photocatalytic activity in 2D layered heterostructures with type II band alignment.(10) In particular, a photocatalytic efficiency of 1% is predicted in MoS2-ZnO and WS2-ZnO layered heterostructures, in which MoS2 (or WS2) predominantly contributes to the conduction band minimum (CBM) and the ZnO layers dominate the valence band maximum (VBM).(10) This physical separation of photoelectrons and holes in different layers of a heterostructure significantly reduces charge recombination. Furthermore, the reduced bandgaps of these heterostructures, 1.60 eV for MoS2-ZnO and 2.05 eV for WS2-ZnO, promote more efficient visible light absorption than bulk ZnO.
To exploit the emergent electronic and photocatalytic properties of layered heterostructures in various applications, reliable and scalable synthesis approaches must be developed.(11) Existing methods suffer from low reproducibility and are labor intensive. For example, the most common synthesis approach to produce van der Waals heterostructures from naturally layered materials with relatively weak van der Waals interactions between the layers involves exfoliation and manual stacking of layers of unlike materials. This method was widely used in proof-of-concept experiments, but cannot be scaled up due to poor control over the mutual orientation of the layers(12) This approach is also not applicable to the synthesis of heterostructures where at least one component has ionic or covalent bonding in all three crystallographic directions. Synthesis of 2D layers of materials that are three-dimensional (3D) at standard conditions can be divided into two categories: 1) chemical vapor deposition on metal substrates and 2) hydrothermal synthesis in the presence of ligands. Chemical vapor deposition furnishes high level of control over the quality and structure of layered materials.(12) A notable application of this method was the synthesis of graphene-like ZnO (g-ZnO) on various metal substrates.(13) The limitations of chemical vapor deposition, however, include difficulties in lifting the 2D layers off the substrate and poor scalability. In contrast, hydrothermal synthesis is highly scalable. 2D ZnO produced under mild hydrothermal conditions in aqueous solutions showed the presence of single layers and bi-layers of ZnO in the samples, observed by high resolution transmission electron microscopy (HRTEM) imaging.(14; 15) Thus, the hydrothermal method furnishes an attractive alternative approach for synthesizing multilayer semiconductors for functional devices.
This study proposes a solution-based approach, in which 2D dichalcogenide layers template the growth of metal oxides to promote the development of layered architectures.(16) Thereby, realizing the predicted solution for the limitations of three-dimensional semiconductors, that lies in reducing the intrinsic dimensionality of bulk metal oxide to graphene-like sheets. The resulting quantum confinement is expected to produce heterostructures with type II band alignment and enhance the lifetime of photogenerated electron and holes. The choice of ZnO and MoS2 for the proof-of-concept synthesis was determined based on the known photocatalytic properties of each individual component and the predicted improved performance of the heterostructures. Specifically, graphene-like or multilayer ZnO heterostructures with transition metal dichalcogenides are predicted to have an enhanced photocatalytic activity in the visible to UV range.(17) To address this major synthetic challenge, we utilize 2D templated ZnO growth with MoS2 nanosheets produced via a liquid phase ultrasonic exfoliation method. The ZnO-MoS2 hybrid structure is then synthesized by stabilizing the intermediate phase of Zn(OH)2 plates on MoS2 flakes followed by thermal annealing. The hybrid structure exhibits enhanced photocatalytic activity toward the degradation of an organic dye. The superior performance over pristine MoS2 and bare ZnO nanoparticles can be attributed to the enhanced separation efficiency of photogenerated charge carriers. The facile, low-cost, and low-temperature solution-based method reported here exemplifies a strategy of hybridizing nanoscale 2D exfoliated defect-free nanosheets with a wide selection of semiconductor nanostructures and furnishes a new paradigm for the synthesis of functional materials for clean energy applications.
In this study, ZnO was synthesized in a pH buffer using zinc nitrate as the Zn2+ precursor and HMTA as a weak base. During the reaction, HMTA slowly decomposes in the heated aqueous solution to yield ammonia and formaldehyde as the initial reactants. These in turn result in the formation of hydroxide ions (Eq. 1-2):
The introduced OH-groups can bind to the MoS2 surface, forming a S-Mo-S-OH layer. The MoS2-OH layer then acts as a reactive template for heterogeneous nucleation of zinc hydroxides through the reaction of hydroxide ions with Zn2+(Eq. 3). The morphology of the prepared binary composite MoS2/Zn(OH)2/ZnO at different MoS2 content (Figure 1A,B and Figure S1) changes from irregular thin plates to hexagonal plates with larger surface area and thickness was MoS2 content increases from 1% to 5%. At 1% MoS2, a mixture of large and small plates with thicknesses from 200-300 nm was observed (Figure 1A). When MoS2 content was increased to 5%, regular larger plates with a thickness of ~500 nm was observed (Figure 1B), highlighting the role of the bilayer MoS2-OH in directing the formation of large surface area Zn(OH)2/ZnO plates. XRD analysis was conducted to identify the composition of the samples, shown in Figure 1C. The (102) peak of MoS2 (dashed line) is observed in the XRD pattern of the composite and slightly downshifts as the molar ratio of MoS2 in the composites increases, indicating binding between MoS2 and Zn(OH)2/ZnO. The conclusion is further supported by the results of control experiments, in which MoS2 was absent from the system (Figure S2A), the HMTA was replaced with NaOH (Figure S2B), or the mixed IPA-aqueous solvent was replaced with pure water (Figure S2C). In all these control experiments only pure ZnO twin structures or nanoparticles were obtained, as evidenced by the characteristic XRD pattern (Figure 1C, green, purple and yellow curves). Thereby, the synergistic effect of Zn(NO3)2 and HMTA in a 200:20 ratio, IPA-water mixed solvents, and MoS2 is crucial for the formation of a 2D heterostructure (see SI for details, Figures S3-S6). Furthermore, in the presence of MoS2, increasing of concentration of HMTA from 50mM to 200mM resulted in the formation of zinc hydroxonitrate Zn5(OH)8(NO3)2(H2O)2 in addition to Zn(OH)2 and ZnO phases. The observed formation of phases with higher OH content is likely due to the enhanced release of OH groups produced during HMTA decomposition and enhanced functionalization of MoS2. The hydroxylated S-Mo-S-OH template likely stabilizes intermediate phases and inhibits their conversion into ZnO. Previous studies of ZnO growth in the presence of HMTA revealed that HMTA has a pH buffering effect and that the maximum rate of growth was achieved when HMTA supersaturation slightly exceeded that of Zn(NO3)2.(18) Our experiments showed a similar trend. Further increasing the HMTA concentration to 500 mM at a fixed Zn(NO3)2 content resulted in a higher fraction of ZnO than the Zn5(OH)8(NO3)2(H2O)2 phase (Figures S3 and S4). On the other hand, decreasing Zn(NO3)2 concentration from 200 mM to 20 mM at constant HMTA content in the absence of MoS2 led to the formation of pure ZnO twinned crystals (Figures S5 and S6). Under the same conditions in the presence of a MoS2 templating surface, the fraction of ZnO decreased and that of Zn(OH)2 increased (Figures S5 and S6), supporting the proposed mechanism of the nucleation of an intermediate 2D ZnO phase on a S-Mo-S-OH template.
The final stable ZnO-MoS2 composite phase was obtained by thermal treatment of the Zn(OH)2 /ZnO /MoS2 at 350 °C. Annealing did not alter the original layered structure of the composites observed in SEM micrographs (Figure 2,A1-D1). No intermediate phase was seen in the XRD patterns for all of the annealed products (Figure 2F), confirming the thermal conversion of Zn5(OH)10·2H2O to pure ZnO. High-resolution SEM images (Figure 2,A2-D2) indicate that the ZnO plates are not homogeneous and are composed of different sized grains sintered together to form a relatively compact plate. The surface grain size, calculated from the SEM images (Figure 2,A3-D3), is smaller for the ZnO-MoS2 heterostructure than samples synthesized without MoS2. The grain size difference further illustrates the templating effect of MoS2 on the nucleation and growth of ZnO.
The structure of the obtained ZnO-MoS2 composite was analyzed using HRTEM. The imaging revealed large surface area (0001) ZnO plates on the surface of the heterostructure (Figure 3A, inset). Analysis of the atomic structure of the surface layers identified regions with an interplanar spacing of 0.27 nm and 0.16 nm, ascribed to the (110) facet of MoS2, and regions with an interplanar distance of 0.26 nm, ascribed to the (001) facet of ZnO. The structure of the ZnO-MoS2 plates was further analyzed using cross-sectional S/TEM imaging (Figure 3C,E), revealing the presence of periodically alternating layers. The micrograph shows a layered structure of interchanging regions with characteristic ZnO (110) and MoS2 (110) spacings where each region is composed of 66 ± 8 and 9 ± 2 layers, respectively. As shown in Figure 3D and F, the ZnO (110) 2D regions are not perfectly parallel to each other, but form a relative angle varying between 0.9º and 4.5º. EDS analysis of the compositional distribution in the cross-section area further supports the presence of semi-parallel 2D ZnO regions interlayered with several layers of MoS2. EDS mapping shows that areas containing higher ZnO density have less MoS2 and that MoS2 is absent in regions with no ZnO (Figure 3G, areas marked with the red and white rectangles, respectively). Combined, these data confirm the formation of a layered heterostructure with interchanging 2D regions of ZnO and MoS2. The alignment of the ZnO (110) and MoS2 (110) facets in the cross-section of heterostructure suggest an in-plane alignment of ZnO and MoS2 along the (001) direction. Although the TEM data are insufficient to make definitive conclusions about the details of the lateral alignment between the ZnO and MoS2 layers, the observed in-plane orientation is consistent with epitaxy between the components of the heterostructure. Specifically, the lattice parameters of ZnO and MoS2 in the a- and b- directions are almost identical and equal to a = b = 3.24 Å for ZnO and a = b = 3.19 Å for MoS2, respectively. This 1.5 % lattice mismatch supports the formation of epitaxial heterostructures along the (001) direction, however, further studies are needed to confirm the epitaxial matching between the layers.
Based on these results, two possible pathways to heterostructure formation can be proposed. One possibility is the direct nucleation of ZnO or Zn(OH)2 on functionalized MoS2-OH flakes followed by the assembly of ZnO/MoS2 flakes into a layered structure. Alternatively, ZnO can epitaxially grow in the confined space between MoS2 flakes. In both scenarios HMTA and IPA serve as pH regulating agents and sources of OH-groups(18) for the formation of ZnO and functionalization of the MoS2 surface. Current ex situ studies do not provide sufficient evidence to delineate between these pathways and future studies will focus on elucidating the mechanism of ZnO nucleation on MoS2.
The photocatalytic activity of the layered ZnO-MoS2 heterostructure was investigated using the photocatalytic degradation of Rhodamine B (RB) as a model reaction. The catalyst and dye solutions were aged in the dark for 30 min to establish the adsorption/desorption equilibrium between the dye and ZnO-MoS2 catalyst prior to irradiation with visible light. The adsorption spectra of RB were recorded during the 90 min irradiation of the sample under a 150W Xe-lamp (Figure 4A-D). The intensity of the main absorption peak located at 550 nm continuously decreased as irradiation time increased. The RB solution gradually became colorless during irradiation, indicating the gradual degradation of the dye. Comparing the photodegradation extent of RB in the presence of pure Zn(OH)2, pure ZnO, Zn(OH)2 mixed with MoS2, ZnO mixed with MoS2, Zn(OH)2-ZnO-MoS2, and the ZnO-MoS2 heterostructure revealed an almost 50% improvement in the photocatalytic efficiency of the layered heterostructure compared to that of the pure components or their mixtures (Figure 4E).
The observed significant improvement in the photocatalytic efficiency of the epitaxial ZnO-MoS2 heterostructure is in line with theoretical predictions for layered ZnO-MoS2. Density functional theory simulations predict that ZnO interfacing MoS2 retains its wurtzite structure if the ZnO is at least 4 monolayers thick.(19) These epitaxial ZnO-MoS2 interfaces are predicted to have a type II band alignment with the CBM associated with Mo 4d states of MoS2, a VBM mostly with O 2p states of ZnO with a small contribution from Mo 4d, and a bandgap of 1.33 eV (Figure 4F). (10; 19; 20) This band alignment promotes the physical separation of photogenerated electrons and holes in MoS2 and ZnO phases, respectively, thereby reducing the probability of charge recombination. The second factor that could affect the photocatalytic activity of the heterostructure is the higher specific surface area of the photocatalyst compared to the corresponding pure phases or previously reported core-shell structures.(21) As seen from Figures 3E and 3B, the relatively thin and small surface area MoS2 nanosheets form an epitaxial heterostructure of relatively high surface area with the extended ZnO nanosheets. The extended layered structure enables a higher adsorption of the dye molecules on the heterostructure surface with a resulting enhancement in photocatalytic response. Together, the enhanced specific surface area and lower photoelectron/hole recombination of the catalyst promote more efficient generation of the active species, such as O2-, •OH , e-, and h+, for dye decomposition. It can be inferred from these data that the strong interaction between ZnO and MoS2 sheets in the nanocomposite promote efficient charge separation and facilitate charge transfer.
In this work, a scalable hydrothermal method was developed for the synthesis of a 2D layered ZnO-MoS2 heterostructure through stabilizing intermediate states of Zn(OH)2 on S-Mo-S-OH. Characterization of the structure and morphology indicates that the MoS2 flakes are intercalated between large size ZnO plates to form an epitaxial layered heterostructure. The resulting quantum confinement and band alignment was shown to improve the photocatalytic efficiency of the layered material by almost 50%, compared to the corresponding single-phase catalysts and mixtures. The proposed synthetic approach furnishes a new general method for the scalable and reproducible synthesis of 2D layered heterostructures through stabilizing intermediate states.
Chemicals
MoS2 powder (99%). Zinc nitrate hexahydrate (Zn(NO3)2·6(H2O)), Hexamethylenetetramine (HMTA); Isopropyl Alcohol (IPA). All reagents were of analytical grade and used without further purification.
MoS2 synthesis and exfoliation
Bulk MoS2 powder was dispersed in isopropyl alcohol at a concentration of 40 mg/mL. The prepared MoS2 suspension was sonicated for 2.5 h in a probe sonicator. The dispersion was subsequently centrifuged at 2000 rpm for 30 min to remove unexfoliated MoS2. The obtained supernatant was further centrifuged at 1500 rpm for 15 min. The resulting MoS2-IPA dispersion was used for heterostructure synthesis.
Fabrication of Zn(OH)2-MoS2 and ZnO-MoS2 heterostructures
In the main experiment, a 200mM Zn(NO3)2 solution and a 20mM HMTA solution were prepared by dissolving powders in DI water. A mixture of aqueous solutions of 1 mL Zn(NO3)2 (200mM) and 1mL HMTA (20mM) was added into the exfoliated MoS2-IPA dispersion and aged in the preheated oven for 24 hours at 60°C. The resulting solution was centrifuged with water and ethanol twice and redispersed in ethanol before characterization. For the control experiments, we synthesized a continuously variable series of ZnO-Zn(OH)2 structures by varying the precursor concentration, solvents, temperature and time as well as the amount of MoS2.
Characterization
Powder XRD of all as-prepared samples was performed on a Philips X’PERT Multi-Purpose Diffractometer (MPD) (PANAlytical) equipped with Cu Kα radiation operating at 50 kV and 40 mA within the 2θ range from 5° to 100°. SEM imaging was conducted on the FEI Helios NanoLab 600i dual-beam focused ion beam precision manufacturing instrument operating at 5 kV. Before SEM imaging, all samples were sputter-coated with a thin layer of carbon (∼5 nm) to ensure good conductivity and imaging. Sample suspensions were dropped on the holey carbon-coated copper grids (Lacey Carbon, 300 mesh; Ted Pella, Inc.) for TEM (FEI, Titan, 300 kV), STEM-HAADF characterization, and energy dispersive X-ray spectrometer analysis.
Supporting information
Supporting information is available from the Wiley online Library or from the author.
Acknowledgements
All SEM, TEM and computational work were performed at EMSL, the Environmental Molecular Sciences Laboratory, which is an Office of Biological and Environmental Research user facility located at PNNL. PNNL is operated by Battelle for the Department of Energy under Contract No. DEAC05-76RLO1830.
Funding
This research was performed at Pacific Northwest National Laboratory (PNNL) with support from the Department of Energy, Office of Science, Basic Energy Sciences, Division of Materials Sciences and Engineering, under Award KC020105- FWP12152.
Author Contributions
Lili Liu, Maria L. Sushko, James J. De Yoreo and Jun Liu conceived the idea, designed the research; Lili Liu synthesized all related nanostructures and performed the characterization work, including TEM, SEM and XRD; Ping Chen and Xin Zhang performed the photocatalysis experiments; Libor Kovarik help perform STEM characterization; Lili Liu and Maria Sushko wrote the main manuscript text; All authors reviewed the manuscript.
Conflict of interest
The authors declare no conflict of interest.
Data Availability Statement
The data that supports the findings of this study are available in the supplementary material of this article.
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No competing interests reported.
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Table of Contents Epitaxial two-dimensional heterostructures of metal oxide and transition metal dichalcogenide, that physically separates photoinduced electrons and holes in different layers, are synthesized using novel and highly scalable solution-based approach. The heterostructures demonstrate an almost 50% enhancement in photocatalytic efficiency compared to the corresponding single-phase materials or their mixtures due to the suppression of charge recombination.
